How to view clojure docs in Emacs Cider

Developing clojure with Emacs Cider is great. To be able to view summaries for clojure functions and special forms, type this in you Cider REPL:

(use 'clojure.repl)

Afterwards, you can do stuff like this:

my.repl> (doc conj)
([coll x] [coll x & xs])
conj[oin]. Returns a new collection with the xs
'added'. (conj nil item) returns (item). The 'addition' may
happen at different 'places' depending on the concrete type.

How to automatically refresh Cider when saving a clojure file in Emacs

Emacs has a great mode for using Clojure called Cider. Cider comes with an interactive REPL. The REPL allows you to test your code, start web apps, if you are using Luminus, and all in all accelerates  development. One annoying thing though is that you have to refresh Cider every time you saved your sources. The good thing is that you can do this automatically. Just add the following to your init.el. I took the function from a blog post by Kris Jenkins:

(add-hook 'cider-mode-hook
'(lambda () (add-hook 'after-save-hook
'(lambda ()
(if (and (boundp 'cider-mode) cider-mode)

(defun cider-namespace-refresh ()
"(require '

(define-key clojure-mode-map (kbd "C-c C-r") 'cider-namespace-refresh)

Additionally, you can now run the refresh command manually, by hitting C-c C-r.

Update: I changed the after-save-hook to only trigger if we are in a buffer that has cider-mode enabled. Otherwise every save command in Emacs would have triggered the refresh!

Improved handling of background GNU Global update

Earlier this year I detailed how to run gtags of GNU Global when Emacs is idle. However, this collides with some special Emacs modes, like ediff. That mode makes its own Emacs frame layout, which can be destroyed by the xgtags incremental update buffer. So I conceived a way to disable gtags temporarily as long as ediff is active:

;; This stuff is only needed, if you haven't got xgtags loaded yet
(autoload 'gtags-mode "gtags" "" t)
(require 'xgtags)
(add-hook 'c-mode-common-hook (lambda () (xgtags-mode 1)))
(defun global-update-incrementally () (shell-command "global -u -q" "*Messages*" "*Messages*") )

;; Call gtags update when idle for some time
(defcustom my-gtags-update-idle-time 60
"Number of idle seconds before an incremental gtags update is launched"
:group 'my-group
:type 'integer

;; initially allow gtags updates
(setq my-gtags-update-active t)

(run-with-idle-timer my-gtags-update-idle-time t
(lambda ()
(if (and my-gtags-update-active
(not (minibufferp) )
(message "Running gtags...")

(add-hook 'ediff-quit-hook
(lambda ()
(message "Activating gtags update.")
(setq my-gtags-update-active t)

(add-hook 'ediff-before-setup-hook
(lambda ()
(message "Deactivating gtags update.")
(setq my-gtags-update-active nil)

An argument for Emacs window-local variables

The Emacs developers used argue that window-local variables are not neccessary, since there are indirect buffers, which can do mostly the same thing. In recent Emacs releases, the window state is anyway mostly hidden from Lisp. However, I think sometimes it is useful to have window-local variables, instead of buffer-local variables. Emacs is very buffer-centric, but sometimes the UI and the user’s view is rather window-centric. Two use-cases for window-local variables are:

I am using ECB, and it would be neat to enable or disable minor modes according to the ECB window I am in. This is also useful for people who manually tile their Emacs frame. In the main edit window, I like to have the tabbar, but in my compilation window, I’d rather not.

The buffer stack for a navigation history definitely should be window specific. Everything else would be very confusing. If a window is split, the new window will inherit its parents history.

Switching to a Java package implementation in Emacs

There already quite a few methods to switch between header and implementation of C and C++ sources in Emacs. Maybe I will put one of those here as well later. But now I needed something different: In a big project, I would like to point at a line like import; and Emacs automatically visits the file where this class has been implemented. I wrote two small Elisp functions to perform this:

(defun my-switch-to-java-implementation (packagename classname)
"Tries to switch to the java source file containing the package packagename and
the class classname."
(format "%s %s -iname '*.java' | xargs egrep -l '%s' | sort -u | xargs egrep -l 'class %s' | head -n 1"
(current-buffer) )
(if (= (count-lines (point-min) (point-max) ) 1)
(search-forward "n")
(replace-match "")
(find-file (buffer-string))
(message "Could not find suitable implementation.")

(defun my-switch-to-java-implementation-at-point ()
"Tries to switch to the java source file which contains the package and class imported
in the current line."
(if (looking-at "^import \(.*\)\.\(.+\);$")
(let ( (packagename (buffer-substring (match-beginning 1) (match-end 1) ) )
(classname (buffer-substring (match-beginning 2) (match-end 2) ) ) )
(my-switch-to-java-implementation packagename classname)
(message "This is not an import")

You basically call my-switch-to-java-implementation-at-point (e.g. by binding it to a key) when you are on the import line. Emacs then launches find and grep to look for the definition of your class. Of course, using something like GNU Global would be faster, but also a bit more tricky, since it does not use the packagename, but rather only the class name, which might be used multiple times in your project.
Also note that you have to set the path to your Java source files. I suggest you make a customizable variable of that, or use your ede-project-root or something similar.

Commenting out line or region in Emacs

What I always wanted: commenting or uncommenting a line or region using emacs. Of course, there is comment-region, but this is much nicer:

(defun comment-or-uncomment-line-or-region ()
"Comments or uncomments the current line or region."
(if (region-active-p)
(comment-or-uncomment-region (region-beginning) (region-end))
(comment-or-uncomment-region (line-beginning-position) (line-end-position))

(define-key c-mode-base-map (kbd "C-/") 'comment-or-uncomment-line-or-region)

See also the corresponding Stackoverflow question.

A nice C++ autocomplete configuration for Emacs

Update: I have written a new blog post about this, with updated information and using new and current packages.

Autocompletion for C++ in Emacs is a hot topic — at least for Emacs users. At least for Emacs users who code C++… So here is my setup that I use, and which works reasonably well. I use the autocomplete package with the autocomplete-clang extension.

You can get the autocomplete package at github and the clang extension is available also on many repositories on github I used the one by brianjcj. Under OS X I use clang 3.1, under Linux I use clang 3.0. You may have to point autocomplete to the correct binary via the customizable variable ac-clang-executable.

So basically you have to clone the autocomplete package, clone the clang extension and copy the auto-complete-clang.el file into the same folder. I used ~/bin/emacs/auto-complete, but you can choose any directory. Maybe you have your own site-lisp folder.

(defcustom mycustom-system-include-paths '("./include/" "/opt/local/include" "/usr/include" )
"This is a list of include paths that are used by the clang auto completion."
:group 'mycustom
:type '(repeat directory)

(add-to-list 'load-path "~/bin/emacs/auto-complete")
(require 'auto-complete-config)
(add-to-list 'ac-dictionary-directories "~/bin/emacs/auto-complete/ac-dict")
(require 'auto-complete-clang)
(setq clang-completion-suppress-error 't)
(setq ac-clang-flags
(mapcar (lambda (item)(concat "-I" item))

(defun my-ac-clang-mode-common-hook()
(define-key c-mode-base-map (kbd "M-/") 'ac-complete-clang)

(add-hook 'c-mode-common-hook 'my-ac-clang-mode-common-hook)

The key binding to M-/ is useful on US-keyboard, you may change that to your liking. So to start autocompletion, just type something and hit M-/. Clang will try to parse your file up to point and give useful completions. If it cannot parse your source, the cursor at point will turn red, and the minibuffer will show the error message. Most likely you forgot to add some include paths to mycustom-system-include-paths, or you really do have a syntax error.

The customizable variable mycustom-system-include-paths can be named arbitrarily. Many thanks to the helpful people over at stackoverflow for supplying me with the answer how to make a directory list nicely customizable.

Limiting the number of lines in the compilation window

For large projects, the output in the compilation window can become quite large. Furthermore, I use the ANSI coloring functionality, so that colored build systems look nice. These two things together can slow down Emacs significantly. However, when the compilation buffer gets so long, the output is most probably only success messages anyway. The errors would already have stopped the compilation process. So I wrote the following function to trim the compilation buffer:

(defun my-limit-compilation-buffer ()
"This function limits the length of the compilation buffer.
It uses the variable my-compilation-buffer-length to determine
the maximum allowed number of lines. It will then delete the first
N+50 lines of the buffer, where N is the number of lines that the
buffer is longer than the above mentioned variable allows."
(let ((num-lines (count-lines (point-min) (point-max))))
(if (> num-lines my-compilation-buffer-length)
(let ((beg (point)))
(goto-char (point-min))
(- num-lines my-compilation-buffer-length)
(/ my-compilation-buffer-length 10) ) )
(delete-region (point-min) (point))
(goto-char beg)

Of course, now you still have to register this function to be actually applied to the compilation buffer, so I read about customization variables. And out came this, to introduce a customizable variable for toggling the compilation-trimming behaviour:

(defun set-my-compilation-buffer-length (symbol value)
(set-default symbol value)
(if value
(add-hook 'compilation-filter-hook 'my-limit-compilation-buffer)
(remove-hook 'compilation-filter-hook 'my-limit-compilation-buffer)

(defcustom my-compilation-buffer-length 2500
"The maximum number of lines that the compilation buffer is allowed to store"
:set 'set-my-compilation-buffer-length)

This is my first shot at such “complex” Emacs Lisp functions, so maybe it’s not optimal, and I don’t know much about the Emacs Lisp API either. Improvement suggestions are welcome!
Update: Over at Stackoverflow someone pointed me to a function that is already built into Emacs. So here’s the alternative solution:

(add-hook 'compilation-filter-hook 'comint-truncate-buffer)
(setq comint-buffer-maximum-size 2000)

This function is used for other comint buffers as well, such as the shell.